Abrupt alteration of Asteroid 2004 MN4's spin state during its 2029 Earth flyby

We predict that when Asteroid 2004 MN4 passes 5.6±1.4 Earth radii from Earth's center on April 13, 2029, terrestrial torques during the flyby will alter its spin state in a dramatic manner that will be observable using groundbased telescopes. Although the asteroid will most likely not undergo catastrophic disruption, it may be subject to localized failure across its surface and interior, providing a unique opportunity to measure otherwise inaccessible mechanical properties of an asteroid.     

Evolution of NEO rotation rates due to close encounters with Earth and Venus

In this paper we study the statistical effect of planetary flybys on the rotation rates and states of Near Earth Objects (NEOs). Our approach combines numerical and analytical methods within a Monte Carlo model that simulates the evolution of the NEO spin rates. We take as input for the simulation a source distribution of spin states and evolve it to find their steady state distribution. In performing this evolution we track the changes in the spin rate and state distribution for the different components of the NEO population. We show that the cumulative effect of planetary encounters is to spin up the overall population of NEOs. This spin up effect holds on average only, and particular members of the population may experience an overall decrease in rotation rate. This effect is clearly seen across all components of the NEO population and is significant both statistically and physically. For initially slow rotators the spin up effect is strong, lowering the mean rotation period by 32%. For faster rotating populations the effect is less, lowering the spin period by 15% for the intermediate case, 6% for fast rotating rubble piles, and 8% for fast rotating monoliths. Physically, the spin up effect pushes 1% of the fast rotating rubble-pile NEOs over the disruption limit, while 6% of these bodies experience a sub-disruption event that could modify their physical structure. For monolithic NEOs, the spin up effect is self-limiting, reaching a minimum spin period of 1.1 hr, with a strong cut-off between 2-3 hr. This has two implications. First, it may not be necessary to invoke the rubble-pile hypothesis to recover a cut-off in spin period. Second, it shows that planetary flybys cannot account for the extremely rapid rotation rates of some small NEOs. We also tested a different balance between the effects of Earth and Venus by treating the Aten sub-class of asteroids separately. Due to increased interactions with the planets, the spin up effect is more pronounced (10%) and disruptions increase by a factor of three. The slow rotation tails of the spin distributions are increased to longer periods, in general, with rotation periods of over 100 hr occurring for a few tenths of a percent for some component populations. Thus, this mechanism may account for some of the noted excess in slow rotators among the NEOs. Planetary flybys also cause NEOs to enter a tumbling state, with approximately 0.5% of the population being placed into a long-axis rotation mode. Finally, based on the evolution of spin states of different components of the NEO population, we compared the evolved states with the measured distribution of NEOs to estimate the relative populations of these components that comprise the NEOs.     

全球海洋对极移周年运动的激发

目前海洋对极移季节变化的激发尚未得到合理的定量结果.许多研究已经表明大气运动是极移季节变化最大的激发源,海洋运动是剩余部分最主要的激发源之一.利用新一代SODA海洋同化资料(SODA_1.4.2和SODA_1.4.3)以及ECCO海洋同化资料,深入研究了1992—2004年全球海洋对极移周年变化的激发以及激发随纬度的分布.结果表明,SODA海洋激发的季节变化与从测地激发函数中扣除大气、陆地水作用剩余部分的季节变化在所研究的大部分时段非常接近,二者的周年振幅和位相结果基本相当.此外,与早期SODA_Beta 7的结果比较,新一代的SODA海洋激发有了明显改善.SODA和ECCO海洋对极移周年激发的纬度分布,在格林威治方向上比较一致;在东经90°方向上有明显的差别.     

Convection in the Presence of an Inclined Axis of Rotation with Applications to the Sun

Thermal convection in a horizontal fluid layer heated from below and rotating about an arbitrary axis is studied analytically with the attention focused on mean flows and drifts generated by the convection velocity field. Mean flows occur in both horizontal directions when the angle between the rotation vector and the vertical is finite but less than 90°. In the case of a hexagonal convection pattern, a wavelike drift is found in the presence of a horizontal component of rotation. Applications to solar convection are discussed. Considering the simplicity of the model the agreement with observations is surprisingly good.     

Statistical analysis of C and S Main Belt Asteroids

In this paper we compare the observable properties of 962 numbered MBAs (Main Belt Asteroids) of Tholen/SMASSII C and S class, with diameter in the range 1-500 km, not belonging to families or binary systems. Above 20 km, the diameters distributions of C and S are similar while under 20 km there is a clear observative bias in favour of small S asteroids which prevents a direct comparison. There is a significant correlation between rotation frequency and diameter both for C and S: if the diameter decreases the rotation frequency tends to increase. There is also a significant correlation between the lightcurve amplitude and the diameter for both samples: if the diameter decreases the lightcurve amplitude tends to increase. For larger diameter the C amplitude tends to be systematically higher than S amplitude of about 0.1 magnitude, but the difference is not very significant. Between 48 and 200 km, the C asteroids have a rotation frequency distribution compatible with a Maxwellian. On the other side, for S asteroids, the compatibility with the Maxwellian concerns diameters greater than 33 km. Considering the rotational properties and the lightcurve amplitude it appears that there are no substantial differences between the samples of C and S asteroids taken into account, and this indicates a good homogeneity in the processes of collisional evolution.     

Zero secular torque on asteroids from impinging solar photons in the YORP effect: A simple proof

YORP torques, where “YORP” stands for “Yarokovsky-O’Keefe-Radzievskii-Paddack,” arise mainly from sunlight reflected off a Solar System object and the infrared radiation emitted by it. We show here, through the most elementary demonstration that we can devise, that secular torques from impinging solar photons are generally negligible and thus cause little secular evolution of an asteroid’s obliquity or spin rate.     

A possible YORP effect on C and S Main Belt Asteroids

A rotating frequency analysis in a previous paper, showed that two samples of C and S-type asteroids belonging to the Main Belt, but not to any families, present two different values for the transition diameter to a Maxwellian distribution of the rotation frequency, respectively 48 and 33 km. In this paper, after a more detailed statistical analysis, aiming to verify that the result is physically relevant, we found a better estimate for the transition diameter, respectively D_C = 44 ± 2 km and D_S = 30 ± 1 km. The ratio between these estimated transition diameters, D_C/D_S = 1.5 ± 0.1, can be supported with the help of the YORP (Yarkovsky-O’Keefe-Radzievskii-Paddack) effect, although other physical causes cannot be completely ruled out.In this paper we have derived a simple scaling law for YORP which, taking into account the different average heliocentric distance, the bulk density, the albedo and the asteroid “asymmetry surface factor”, has enabled us to reasonably justify the ratio between the diameters transition of C-type and S-type asteroids. The same scaling law can be used to estimate a new ratio between the bulk densities of S and C asteroids samples (giving ρ_S/ρ_C ≈ 2.9 ± 0.3), and can explain why the asteroids near the transition diameter have about the same absolute magnitude. For C-type asteroids, using the found density ratio and other estimates of S-type density, it is also possible to estimate an average bulk density equal to 0.9 ± 0.1 g cm⁻³, a value compatible with icy composition. The suggested explanation for the difference of the transition diameters is a plausible hypothesis, consistent with the data, but it needs to be studied more in depth with further observations.     

Using sparse photometric data sets for asteroid lightcurve studies

With the advent of wide-field imagers, it has become possible to conduct a photometric lightcurve survey of many asteroids simultaneously, either for that single purpose (e.g., Dermawan, B., Nakamura, T., Yoshida, F. [2011]. Publ. Astron. Soc. Japan 63, S555–S576; Masiero, J., Jedicke, R., ?urech, J., Gwyn, S., Denneau, L., Larsen, J. [2009]. Icarus 204, 145–171), or as a part of a multipurpose survey (e.g., Pan-STARRS, LSST). Such surveys promise to yield photometric data for many thousands of asteroids, but these data sets will be “sparse” compared to most of those taken in a “targeted” mode directed to one asteroid at a time.We consider the potential limitations of sparse data sets using different sampling rates with respect to specific research questions that might be addressed with lightcurve data. For our study we created synthetic sparse data sets similar to those from wide-field surveys by generating more than 380,000 individual lightcurves that were combined into more than 47,000 composite lightcurves. The variables in generating the data included the number of observations per night, number of nights, noise, and the intervals between observations and nights, in addition to periods ranging from 0.1 to 400 h and amplitudes ranging from 0.1 to 2.0 mag.A Fourier analysis pipeline was used to find the period for each composite lightcurve and then review the derived period and period spectrum to gauge how well an automated analysis of sparse data sets would perform in finding the true period. For this part of the analysis, a normally distributed noise level of 0.03 mag was added to the data, regardless of amplitude, thus simulating a relatively high SNR for the observations. For the second part of the analysis, a smaller set of composite curves was generated with fixed core parameters of eight observations per night, 8 nights within a 14-day span, periods ranging from 2 to 6 h, and an amplitude of either 0.3 mag or 0.4 mag. Individual data sets using these fixed parameters added normally-distributed noise of 0.05, 0.1, or 0.2 mag. The analysis examined the success rates for finding the true period as the noise increased towards levels simulating data for objects close to sky background levels.After applying a filter to remove highly-ambiguous solution sets, the best chance for success was found to be when the true period was in the range of P ≈ 2–5 h and amplitudes were A ? 0.5 mag. The solution sets for lightcurves with low amplitude, long periods, and/or those that were sampled too sparsely in comparison to the period were often too ambiguous to be considered reliable for statistical rotation studies. Analysis of slow rotators (P > 24 h) found that somewhat reasonable solutions of P < 6 h could be found for about 15–20% of those objects, even at higher amplitudes, indicating that the Fourier analysis had locked onto the noise in the data.Efforts to produce an automated pipeline to help determine an unambiguous (or nearly so) solution based on the period spectrum from the Fourier analysis were made. These proved unsuccessful because of the number of parameters that must be considered and the difficulties in assigning an objective weight to each one in finding a final result. Despite this initial failure, further attempts will be made to quantify the U rating system.Comparison of the synthetic data analysis results to those from two actual surveys shows a reasonable agreement between the two. A review of the pros and cons of sparse versus dense data sets shows that each has a significant role in future studies and that it will be critical to establish open lines of communications and data exchange between the deep wide-field sparse data surveys and dense data programs.     

Combined effect of YORP and collisions on the rotation rate of small Main Belt asteroids

The rotation rate distribution of small Main Belt asteroids is dominated by YORP and collisions. These mechanism act differently depending on the size of the bodies and give rise to non-linear effects when they both operate. Using a Monte Carlo method we model the formation of a steady state population of small asteroids under the influence of both mechanisms and the rotation rate distribution is compared to the observed one as derived from Pravec et al. (Pravec, P. et al. [2008]. Icarus 197, 497-504). A better match to observations is obtained with respect to the case in which only YORP is considered. In particular, an excess of slow rotators is produced in the model with both collisions and YORP because bodies driven to slow rotation by YORP have a random walk-like evolution of the spin induced by repeated collisions with small projectiles. This is a dynamical evolution different from tumbling and it lasts until a large impact takes the body to a faster rotation rate. According to our model, the rotational fission of small asteroids is a very frequent event and might explain objects like P/2010 A2 and its associated tail of millimeter-sized dust particles. The mass loss during fission of small asteroids might significantly influence the overall collisional evolution of the belt. Fission can in fact be considered as an additional erosion mechanism, besides cratering and fragmentation, acting only at small diameters.     

Photometric observations of 107P/Wilson-Harrington

We present lightcurve observations and multiband photometry for 107P/Wilson-Harrington using five small- and medium-sized telescopes. The lightcurve has shown a periodicity of 0.2979 day (7.15 h) and 0.0993 day (2.38 h), which has a commensurability of 3:1. The physical properties of the lightcurve indicate two models: (1) 107P/Wilson-Harrington is a tumbling object with a sidereal rotation period of 0.2979 day and a precession period of 0.0993 day. The shape has a long axis mode (LAM) of L₁:L₂:L₃ = 1.0:1.0:1.6. The direction of the total rotational angular momentum is around λ = 310°, β = −10°, or λ = 132°, β = −17°. The nutation angle is approximately constant at 65°. (2) 107P/Wilson-Harrington is not a tumbler. The sidereal rotation period is 0.2979 day. The shape is nearly spherical but slightly hexagonal with a short axis mode (SAM) of L₁:L₂:L₃ = 1.5:1.5:1.0. The pole orientation is around λ = 330°, β = −27°. In addition, the model includes the possibility of binary hosting. For both models, the sense of rotation is retrograde. Furthermore, multiband photometry indicates that the taxonomy class of 107P/Wilson-Harrington is C-type. No clear rotational color variations are confirmed on the surface.